This invention relates to a frequency standard generator for easily generating a highly accurate and stabilized reference frequency which is common in all over the world, and more particularly, to a frequency standard generator for providing a high accuracy reference frequency at anytime and anywhere with low cost by receiving a high accuracy time signal from a satellite or a communication network having a high accuracy frequency standard such as an atomic frequency standard or an equivalent thereof.
A recent advanced communication system requires a higher resolution and accuracy clock signal uniformly used in communication networks and systems. In particular, a digital communication network, a data communication network including mobile communication networks and TV broadcasting networks, and an environmental observation network of global scale demand this high resolution clock signal. In the conventional information communication systems, such requirements have been carried out by various configurations as described below.
Generally, an atomic frequency standard is used as a primary standard and a high stability quartz-crystal oscillator is calibrated to the atomic frequency standard to function as a secondary standard. However, with the increase of information volume, the combination of the atomic standard and the quartz-crystal standard has become insufficient in terms of its frequency resolution since the quartz-crystal oscillator is used as a secondary standard. Thus, an atomic frequency standard, especially a rubidium atomic standard is used as a secondary standard to improve the frequency resolution.
In domestic communication networks, it is a common practice to use a frequency information of an upper level station having a higher frequency accuracy for a frequency standard of a lower level station. In one example, the frequency information from the upper level station is distributed to the lower level stations as a reference clock through exclusive lines so that clock signals in the lower level directly synchronize with the reference clock signal. In another example, the frequency information in the upper station is embedded in a data structure and distributed to the lower level stations through data lines, and then the lower level stations detect the reference clock components from the data to synchronize a lower level clock with the reference clock in the data.
In a TV broadcasting network, carrier signals are formed based on a highly stabilized oscillator provided in each broadcasting station. Usually, such a highly stabilized oscillator utilizes a high stability quartz-crystal oscillator housed in a temperature oven. However, regarding a color sub-carrier signal, since net stations must be dependent on key stations, the key stations must maintain a sufficiently high accuracy frequency reference. Therefore, the key stations install rubidium atomic standards to be calibrated to radio frequency signals provided by a standard organization of each country or a Loran C navigation radio wave to maintain the high accuracy.
A carrier signal from a TV broadcasting local station utilizes an offset carrier method in which frequencies of the carrier signals in a given channel are set to be slightly different from each other to obviate an interference within the channel as well as to improve an effective use of the frequency resources. At present, the frequency offset in the local station need not be very precise as required in the key station, and thus the reference clock for forming the offset carrier is not frequently calibrated. However, since a range of frequency allocation per channel is getting more and more smaller, in the near future, the local TV stations may employ a synchronous broadcasting method which requires a high accuracy reference clock.
In the synchronous broadcasting method, carrier signal frequencies between the stations which tend to cause interference problems are precisely synchronized with each other, which obviates the interference problems in the same channel. In this method, like the color sub-carrier, the carrier signals must be precisely synchronized between the stations. Feasibility of the synchronous broadcasting method is also discussed in a radio broadcasting system in which the frequency allocation is even more tighter than that of the TV broadcasting system, although any agreement for a measure of an accurate frequency generation has not reached so far.
For international communication systems, it is recommended by Technical Session of the International Telecommunications Union (ITU/TS) that the reference frequency of each country keep the short term accuracy of 10.sup.-9 and the long term accuracy of 10.sup.-11 with respect to Universal Co-ordinate Time (UTC) which is provided by the International Bureau of Weights and Measures (BIPM). To meet this recommendation, each country installs a cesium atomic frequency standard as a high stability frequency source and, as a means for UTC transfer, utilizes the standard radio waves provided by various countries or Loran C navigation waves. These radio waves are always accurately synchronized with the UTC, and if there exists a frequency offset therebetween, such a frequency offset will be publicly announced.
In a natural and environmental observation network, it is necessary to observe the environmental information throughout the earth, which requires a precise synchronization among the frequencies of oscillators in the observation stations in many places. To satisfy this requirement, a frequency deviation with respect to UTC is constantly monitored by using an available standard such as a standard radio wave.
Recently, the United States Department of Defense has launched a high resolution positioning system called a Global Positioning System (GPS) involving artificial satellites. Each GPS satellite installs a cesium atomic oscillator whose frequency is managed and calibrated by the Department of Defense. The GPS satellite transmits information concerning its position and an accurate time clock, i.e., a GPS time signal. A phase per second in the GPS time signal is synchronized within one microsecond (10.sup.-6) with respect to the UTC. Further, since a phase offset for a time length which is more than one second is known, it is easy to convert the GPS time to UTC.
Since 1993, United States Department of Defense and United States Department of Transportation opened C/A (Coarse/Acquisition) codes of the GPS satellite to private use. Further, the United States government is planning to further improve the positional resolution of this system. At present, twenty six (26) GPS satellites are running around the earth and thus, at least three or four satellites are always accessible from anywhere on the earth.
At present, a frequency standard generator is available in which the GPS time signal is utilized as a primary standard. However, in such a frequency standard generator, a rubidium atomic standard is synchronized with the GPS time signal. Thus, in this device, the rubidium atomic standard is used as a secondary oscillator like the key stations of the TV networks as noted above.
The foregoing conventional frequency standard systems involve disadvantages as described below. Although the system using the atomic frequency standard is a highly accurate frequency standard, it has a disadvantage that it involves a high cost. Not only the system itself is expensive but maintaining the system also requires a high cost since regular replacements of lamps and gas cells are necessary in the system.
In the wire communication network, exclusive lines for sending frequency references must be constructed in addition to the data lines. In case where only the data lines are provided, a source station of a reference frequency has to transform the waveform of the reference in the data and a receiving end has to extract the reference frequency from the data, which degrades the accuracy of the reference frequency. Furthermore, since the wire communication network involves many layers of communication lines and thus the overall communication lines tend to be very lengthy, the synchronization of the clock signals are subject to phase jitters and wanders caused by environments surrounding such communication lines.
The TV broadcasting networks and the international communication networks require a monitoring system to monitor frequency deviations between the standard radio waves and the UTC. These networks further need constant calibration works to maintain the accuracy level of the standard frequency. The synchronized broadcasting systems also need such monitoring systems and regular calibration works in the local stations by using standard radio waves or other frequency standards. The environmental observation systems also require constant calibration works and the like.
As has been in the foregoing, the conventional systems involve various problems including the degradation of synchronization in the layers of communication lines, the needs of the monitoring system of the frequency deviations with respect to the UTC, the needs of the constant calibrations with respect to the UTC, and the needs of the high cost atomic frequency standard which further requires regular maintenance works. Therefore, there is a need to provide a new frequency standard generator which can solve the problems associated with the conventional frequency standard systems.